Technological development and increased demand for mobile equipment have led to rapid increase in the demand for secondary batteries as energy sources. Among these secondary batteries, lithium secondary batteries having high energy density and voltage, long cycle span and low self-discharge are commercially available and widely used.
In addition, increased interest in environmental issues has brought about a great deal of research associated with electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles as alternatives to vehicles using fossil fuels such as gasoline vehicles and diesel vehicles which are main causes of air pollution. These electric vehicles generally use nickel-metal hydride (Ni-MH) secondary batteries as power sources. However, a great deal of study associated with use of lithium secondary batteries with high energy density, discharge voltage and power stability is currently underway and some are commercially available.
A lithium secondary battery has a structure in which a non-aqueous electrolyte comprising a lithium salt is impregnated into an electrode assembly comprising a cathode and an anode, each comprising an active material coated on a current collector, and a porous separator interposed therebetween.
Lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide, lithium composite oxide and the like are generally used as cathode active materials of lithium secondary batteries and carbon-based materials are generally used as anode active materials thereof and use of silicon compounds, sulfur compounds and the like is also considered.
However, lithium secondary batteries have various problems, in particular, problems associated with fabrication and driving properties of an anode.
First, regarding fabrication of an anode, a carbon-based material used as an anode active material is highly hydrophobic and thus has problems of low miscibility with a hydrophilic solvent in the process of preparing a slurry for electrode fabrication and low dispersion uniformity of solid components. In addition, this hydrophobicity of the anode active material complicates impregnation of highly polar electrolytes in the battery fabrication process. The electrolyte impregnation process is a kind of bottleneck in the battery fabrication process, thus greatly decreasing productivity.
In order to solve these problems, addition of surfactant as an additive to an anode, an electrolyte or the like is suggested. However, disadvantageously, the surfactant may have side effects on driving properties of batteries.
Meanwhile, regarding driving properties of anode, disadvantageously, the carbon-based anode active material induces initial irreversible reaction, since a solid electrolyte interface (SEI) layer is formed on the surface of the carbon-based anode active material during an initial charge/discharge process (activation process), and battery capacity is reduced due to exhaustion of the electrolyte caused by removal (breakage) and regeneration of the SEI layer during a continuous charge/discharge process.
In order to solve these problems, various methods such as formation of an SEI layer through stronger bonding, or formation of an oxide layer on the surface of the anode active material have been attempted. These methods have properties unsuitable for commercialization such as deterioration in electric conductivity caused by the oxide layer and deterioration in productivity caused by additional processes. Also, there still exists a problem in that growth of dendrite lithium on the surface of the anode active material may still cause short-circuit.
Accordingly, there is an increasing need for secondary batteries capable of solving these problems.